1. Field of the Invention
The present invention relates to a small watercraft such as a personal watercraft (PWC) which ejects water rearward and planes on a water surface as the resulting reaction. More particularly, the present invention relates to a gas-release structure of the engine. As defined herein, the gas refers to a gas containing mist oil or a fuel, or air.
2. Description of the Related Art
In recent years, so-called jet-propulsion personal watercraft, which are one type of small watercraft, have been widely used in leisure, sport, rescue activities, and the like. The jet-propulsion personal watercraft is configured to have a water jet pump that pressurizes and accelerates water sucked from a water intake generally provided on a bottom hull surface and ejects it rearward from an outlet port. Thereby, the personal watercraft is propelled. In the jet-propulsion personal watercraft, a steering nozzle provided behind the outlet port of the water jet pump is swung either to the right or to the left by operating a bar-type steering handle to the right or to the left, to change the ejection direction of the water to the right or to the left, thereby turning the watercraft to the right or to the left.
In the personal watercraft so configured, a multi-cylinder engine is mounted such that a crankshaft extends along the longitudinal direction of a body. In a wet-sump engine, an oil tank is provided on a bottom portion of a crankcase to be integral with the crankcase to allow the oil tank and the crankcase to have a common chamber, while, in a dry-sump engine, a chamber of the oil tank is formed to be independent of a chamber of the crankcase. In both engines, lubricating oil inside the oil tank is fed to components of the engine that require lubrication by using a feed pump or the like.
In these engines, with reciprocation of a piston and by a blow-by gas from a combustion chamber, a pressure inside the crankcase varies. In order to inhibit the reciprocation of a piston of the engine from being impeded due to a variation in the pressure, it is required that the crankcase communicate with an ambient side to allow the variation in the pressure of the crankcase to be lessened. More specifically, in the case of the wet-sump engine, the crankcase communicates with the ambient side through a breather pipe, or the crankcase communicates with a cam chamber of a cylinder head through a cam chain tunnel and the cam chamber communicates with the ambient side through the breather pipe (involving a breather passage) to allow the variation in the pressure of the crankcase to be lessened.
In the case of the dry-sump engine, as in the case of the wet-sump engine, the above breather mechanism is needed. In addition, in the dry-sump engine, since gas (containing the blow-gas) is mixed in the oil being delivered from the chamber of the crankcase by a scavenging pump, the breather mechanism needs to be provided on the chamber of the oil tank independent of the chamber of the crankcase to allow the gas flowing into the chamber of the oil tank to be released to the ambient side.
In the case of the personal watercraft in which the longitudinal direction of the oil tank corresponds with the longitudinal direction of the watercraft (axial direction of the crankshaft of the engine), oil inside the oil tank moves to a front portion or a rear portion due to inertia force while the watercraft is starting and stopping (accelerating or decelerating). Under this condition, the oil is distributed unevenly inside the oil tank, and thereby, an upstream end portion of the breather pipe or a breather hole of the breather mechanism is clogged with the oil. With the breather hole clogged with the oil, the gas inside the oil tank is compressed temporarily, which is caused by reciprocation of the piston or the blow-by gas. For this reason, the oil flows to an outside of the oil tank together with the pressurized gas.
In other operations of the engine, depending on the state of the oil inside the oil tank, the breather pipe or the breather hole is temporarily clogged with the oil. Under this condition, the gas inside the oil tank is temporarily compressed and the oil flows outside the oil tank together with the pressurized gas.
As a solution, a gas-flow cross-sectional area of the breather pipe or the breather hole may be made larger to inhibit clogging. In actuality, however, the gas-flow cross-sectional area needs to be ten to fifteen times larger than that of a normal breather pipe or the like. In the personal watercraft, it is difficult to provide the breather pipe or the breather hole having such a large gas-flow cross-sectional area in a limited space in the vicinity of the engine.